1. Field of the Invention
This invention relates to optical fiber communication circuits and more particularly to controlled attenuation of the intensity of signals in such circuits.
2. Description of the Prior Art
Connections between two fibers are known which include butting the fibers end to end. Because the location of the core within the cladding is random, it is very difficult to achieve juxtaposition of the centers of the end faces of the two cores, as is needed if the loss in the connection is to be tolerable. Not only do the ends of the two cores have to be juxtaposed perfectly but the axes of the two cores adjacent the connection have to be aligned, i.e. they must not have an inclination to one another.
A known type of connection between two fibers or between one fiber and a device such as a splitter, transmitter or receiver is termed an "expanded beam" connection. In such a connection the beam of light leaving a fiber is expanded to a greater diameter and is collimated for transmission towards another fiber or to a device. If the recipient of the transmitted expanded beam is to be another fiber, the beam is focussed by a lens onto the end of the other fiber, i.e. the diameter of the beam is again reduced. As is known, the expanded beam type of connection is advantageous in that lower losses are readily achieved.
It is known that the efficiency of a circuit decreases with age and that if the power of a signal source is adequate at the beginning of the life of the circuit, later in the life the power may be inadequate. For example, it is known that the output of laser diodes decreases with age. If the power of the signal at the beginning of the life was chosen so that it was still adequate at the end of the circuit's life, components of the circuit might be saturated early in the life. Thus, it is desirable to design the circuit so that the power is adequate at the end of the circuit's life and to build in attenuation as and where necessary to avoid saturation. As the circuit ages the built-in attenuation may be progressively decreased.